Used as reference element is, for example, a silver/silver-chloride electrode, which reaches into a reference electrolyte, for example, KCI, located in an annular chamber around the working electrode. The reference electrolyte can be present, for example, as liquid and/or as gel. The working, or measuring, electrode comprises, usually, a glass tube, which is closed with a glass-membrane facing toward the medium to be measured and filled with a buffer solution. The electrode wire reaches into the buffer solution.
Insofar as the glass-membrane has a very high impedance, the measured potential is very disturbance-susceptible, so that it should, as much as possible, be well shielded.
The reference electrolyte in the annular chamber surrounding the measuring electrode is, in certain respects, suitable as shielding; however, it has, for example, a temperature-dependently variable volume, so that an air chamber can form above the reference electrolyte, surrounding the measuring electrode, whereupon the measuring electrode is not sufficiently shielded in the axial section of the air chamber.
FIG. 3 shows, for purposes of explanation, a combination electrode 61 of the state of the art, which has, between an outer tube 62 and an inner tube 63, an annular reference chamber 64, which is filled with a reference electrolyte. The reference potential is tapped with a reference electrode wire 67, which extends through various seals in the axial direction in the reference chamber. In the upper end of the reference chamber 64 is a so-called compensator 74, which comprises a closed-pore, elastic foam. The foam of the compensator is pressure-dependently compressed, so that, within a working range of the foam, the variable volume of the reference electrolyte can be accommodated. Above the compensator are a silicone caulking 78 and an epoxide resin caulking 79. The described apparatus is disadvantageous for the following reasons. On the one hand, the volume accommodated by the limited compressibility of the foam is limited, and, on the other hand, the volume of the foam cannot be arbitrarily increased, since, in the area of the foam, no effective shielding is provided. Furthermore, problems with sealing can be expected, since the foam moves relative to the reference electrode wire, and such movement can also affect the interface between the silicone caulking and the electrode wire.